Calls incoming to my Google Voice number already ring in Hangouts for me - I actually use that any time I have to do phone conferencing now, since I can just use my computer headset rather than futzing with a phone headset.
Oh god, I love it. My computer even rings a good second or two before my phone so I can hit Pause on my keyboard (Spotify pauses) and then click "Answer". I've got a usb headset with decent speaker-drivers so I can listen to music while I work and switch between Lync/Hangout calls easily.
That's actually good. But the issue I was highlighting was about making calls. Not that Hangouts cannot make a phone call - it's just that I don't want to remember all the numbers of my contacts.